Single-crystal silicon is conventionally used as a material for a high voltage power semiconductor device controlling a high current. Various types of power semiconductor devices exist and each is used for a purpose suitable therefor. For example, a bipolar transistor and an insulated gate bipolar transistor (IGBT) can respectively handle a high current density but these transistors are incapable of high speed switching. Frequency limits for the bipolar transistor and the IGBT are on the order of several kHz and about 20 kHz, respectively. On the other hand, although a power MOSFET (metal oxide semiconductor field effect transistor) cannot handle high current, this transistor can switch at high speeds up to several MHz.
Nonetheless, in the market, demand is strong for a power device that can cope with both high current and high speed, and efforts have been made to improve the IGBT, the power MOSFET, etc. Therefore, at present, development has advanced to the extent that the performance substantially reaches theoretical limitation decided by the materials. FIG. 1 is a cross-sectional diagram of a structure of a common MOSFET. A high concentration n+-source layer d is formed that is selectively formed in a surface layer of a P-base layer c deposited on an n−-drift layer b, and a gate electrode f is formed through a gate insulating film e on the surface of the low concentration n−-drift layer b, the P-base layer c, and the n+-source layer d.
A superjunction MOSFET has recently attracted attention. The theory of the superjunction MOSFET was reported by Fujihira, in 1997 (see, e.g., Non-Patent Literature 1 below) and this MOSFET was established as a product called “CoolMOS” by Deboy, et al., in 1998 (see, e.g., Non-Patent Literature 2 below). The superjunction MOSFET is characterized in that the ON-resistance thereof is significantly improved without degrading the breakdown voltage between the source and the drain, by forming a P-layer in a columnar structure along a vertical direction (the depth direction) in the n−-drift layer.
Investigations concerning the material from the viewpoint of the power semiconductor device have been conducted and Shenai, et al., reported a power semiconductor device using SiC as the semiconductor material (see, e.g., Non-Patent Literature 3 below). SiC has recently attracted attention for next generation power semiconductor devices as it is excellent with regard to low ON-voltage, high speed properties, and high temperature properties. The reason for this is that SiC is a highly stable material chemically, has a wide band gap that is three eV, and can be used very stably as a semiconductor at high temperatures. The critical electric field of SiC is higher than that of silicon by one or more digit(s).
The material performance of SiC can exceed the limit of the material performance of silicon and therefore, the future growth of SiC is highly expected in uses for power semiconductors, especially, MOSFETs. Expectations especially for the low ON-resistance are high, and a vertical SiC-MOSFET is counted on that facilitates further reduction of the ON-resistance maintaining the high voltage.
The cross-sectional structure of a common SiC-MOSFET is the structure depicted in FIG. 1 similarly to that of the silicon. The P-base layer c is deposited on the n−-drift layer b and the n+-source layer d is selectively formed in the surface layer of the P-base layer c (the surface layer on the side opposite to that of the n−-drift layer b). The gate electrode f is formed through the gate insulating film e on the surface of the n−-drift layer b, the P-base layer c, and the n+-source layer d. The n−-drift layer b is deposited on the front face of a substrate “a” and a drain electrode g is formed on the back face of the substrate “a”.
It is expected that the SiC-MOSFET will be utilized, as a switching device, in a power conversion equipment such as a motor control inverter or an uninterruptible power supply (UPS) as a device capable of high-speed switching with low ON-resistance. SiC is a wide band gap semiconductor material; therefore, the critical electric field strength is high and about 10 times that of silicon as above; and may be able to sufficiently reduce the ON-resistance.
For example, in the case of the MOSFET, when high voltage is applied between the source and drain, the high voltage is applied to not only the active region but also to the edge termination structure surrounding the active region. In the edge termination structure, a depletion layer is expanded along the horizontal direction (a direction perpendicular to the depth direction) when the high voltage is applied thereto. Therefore, the edge termination structure tends to be influenced by charges on the device surface and as a result, the breakdown voltage thereof may be unstable. A junction termination extension (JTE) structure published by T. K. Wang, et al, (see, e.g., Non-Patent Literature 4 below) is famous as an edge termination structure for a SiC device.
Patent Documents 1 to 3 below are disclosed as examples that are first disclosed as a Si power device (see, e.g., Non-Patent Literature 5 below) and that are applied to SiC. However, the JTE structure has a disadvantage in that the breakdown voltage significantly varies due to the variations in the impurity concentration in the P-layer. Consequently, this is also a serious problem for Si devices and therefore, it is estimated that the same problem arises for the SiC device.    Patent Document 1: U.S. Pat. No. 6,002,159    Patent Document 2: U.S. Pat. No. 5,712,502    Patent Document 3: U.S. Pat. No. 3,997,551    Non-patent Literature 1: Fujihira, JJAP Vol. 36, Part 1, No. 10, pp. 6254, 1997    Non-patent Literature 2: Deboy, et al, IEEE IEDM 1998, pp. 683    Non-patent Literature 3: IEEE Transaction on Electron Devices, Vol. 36, p. 1811, 1989    Non-patent Literature 4: IEEE ISPSD, 1992, pp. 303-308    Non-patent Literature 5: Temple IEEE Transaction on Electron DeVices, Vol. ED33, Vol. 10, PP. 1601, 1986